Night vision device having an image intensifier tube, microchannel plate and power supply for such an image intensifier tube, and method

ABSTRACT

A night vision device with an image intensifier tube includes an improved power supply which operates the image intensifier tube of the device according to a variable duty cycle either at a design voltage level for the tube or at a voltage level simulating a dark-field. This duty cycle variation is effected as a function of the current flow in the image intensifier tube in order to provide automatic brightness control and bright source protection. An improved service life for the image intensifier tube is achieved. Also, a microchannel plate of the image intensifier tube need not carry an ion barrier film, resulting in an improved light amplification for the image intensifier tube.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation in part of application Ser.No. 08/901,419, filed Jul. 28, 1997, now U.S. Pat. No. 5,949,063.

FIELD OF THE INVENTION

The present invention is generally in the field of night vision devicesof the light-amplification type. Such night vision devices employ animage intensifier tube (I² T) to receive photons from a scene. Thisscene may be illuminated by full day light; or the scene may beilluminated with light which is either of such a low level, or of such along wavelength (i.e., infrared light), or both, that the scene isinvisible or is only dimly visible to the natural human vision. The I² Tresponsively provides a visible image replicating the scene.

More particularly, the present invention relates to an improved nightvision device having an I² T with a low level of ion feedback, and to animproved power supply for an I² T achieving an extended operating life.A method of operating an I² T and a method of operating a power supplyfor such a tube are both disclosed.

BACKGROUND OF THE INVENTION

Even on a night which is too dark for natural human vision, invisibleinfrared light is richly provided by the stars. Human vision cannotutilize this infrared night-time light from the stars because theso-called near-infrared portion of the spectrum is invisible for humans.Under such conditions, a night vision device of the light amplificationtype can provide a visible image replicating the night time scene. Suchnight vision devices generally include an objective lens which focusesinvisible infrared light from the night-time scene onto the transparentlight-receiving face of an image intensifier tube (I² T). At itsopposite image-face, the I² T provides an image in visible yellow-greenphosphorescent light, which is then presented to a user of the devicevia an eye piece lens.

A contemporary night vision device will generally use an I² T with aphotocathode behind the light-receiving face of the tube. Thephotocathode is responsive to photons of infrared light to liberatephotoelectrons. These photoelectrons are moved by a prevailingelectrostatic field to a microchannel plate having a great multitude ofdynodes, or microchannels. These dynodes or microchannels have aninterior surface substantially defined by a material of high emmisivityof secondary electrons. In other words, each time an electron (whether aphotoelectron or an electron previously emitted by the microchannelplate) collides with this interior surface material, more than oneelectron (i.e., secondary-emission electrons) leaves the site of thecollision. This process of secondary electron emissions is not anabsolute in each case, but is a statistical process. The photoelectronsentering the microchannels thus cause a geometric cascade ofsecondary-emission electrons moving along the microchannels so that aspatial output pattern of electrons which replicates an input pattern(but at a considerably higher electron density than the input pattern)issues from the microchannel plate. This pattern of electrons is movedfrom the microchannel plate to a phosphorescent screen electrode byanother electrostatic field. When the electron shower from thephotocathode impacts on and is absorbed by the phosphorescent screenelectrode, a visible image is produced. This visible image is passed outof the tube through a transparent window for viewing.

A conventional power supply for a conventional I² T provides theelectrostatic field potentials referred to above, and also provides afield and current flow to the microchannel plate(s). This power supplyprovides the necessary voltage levels to provide the requiredelectrostatic fields maintained within the image intensifier tube tomove electrons from the photocathode to the screen electrode.Unavoidably, these electrostatic fields also move any positive ionswhich exist within the image intensifier tube toward the photocathode.Because such positive ions may include gas atoms of considerable size,they are able to impact upon and cause damage to the photocathode ofmany conventional image intensifier tubes. This impact of positive ionson the photocathode contributes to a relatively short operating life formany early-generation image intensifier tubes. As those ordinarilyskilled in the pertinent arts will understand, later generation imageintensifier tubes of the proximity focus type have partially solved thision-impact problem by providing an ion barrier film on the inlet side ofthe microchannel plate.

This ion barrier film itself, however, is not without disadvantages. Arecognized disadvantage of such an ion barrier film on a microchannelplate is the decreased gain provided by the microchannel plate between aphotocathode of an image intensifier tube and the output screenelectrode of the tube. That is, the ion barrier film also acts as abarrier preventing some of the photoelectrons liberated from thephotocathode of the tube from reaching the microchannels of themicrochannel plate. In some cases, as much as 50% of the electronsliberated from the photocathode of the I² T and approaching themicrochannel plate will be blocked and will not reach the microchannelsto be amplified as described above. Thus, about the same percentage ofthe image information which theoretically could be provided by this tubeis lost.

U.S. Pat. No. 3,720,535, issued Mar. 13, 1973; U.S. Pat. No. 3,742,224,issued Jun. 26, 1973; and U.S. Pat. No. 3,777,201, issued Dec. 4, 1973provide examples of microchannel plates or image intensifier tubeshaving an ion barrier film on a microchannel plate.

Further to the above, conventional night vision devices (i.e., since the1970's and to the present day) provide several protective functions. Oneof these protective functions is referred to as "automatic brightnesscontrol" (ABC), and another protective function is called "bright sourceprotection" (BSP). The ABC function maintains the brightness of theimage provided to the user substantially constant despite changes in thebrightness (in the infrared and near-infrared portion of the spectrum)of the scene being viewed. BSP prevents the I² T from being damaged byan excessively high current level in the event that a bright source,such as a flare or fire, comes into the field of view. These functionsare somewhat analogous to area and spot exposure controls on a camera.ABC controls the image brightness using the entire area of the viewedscene, while BSP uses light emission levels from a spot in the scene(which need not be centered in the scene) in order to provide theprotective function.

The ABC function is conventionally accomplished by providing a regulatorcircuit monitoring the output current from the phosphorescent screenelectrode (See FIG. 9). When this current exceeds a certain threshold,the field voltage level across the opposite faces of the microchannelplate(s) is decreased to reduce the gain of the microchannel plate(s),as is graphically depicted in FIG. 10. This reduction of microchannelplate voltage also has the effect of reducing the resolution of the I²T. That is, the gain versus voltage function of the I² T at lowered MCPvoltages results in a matrix pattern from the microchannel plate(s)appearing in the image. This matrix pattern is sometimes referred to asfixed-pattern noise in the image.

As a result, in bright-field conditions with the ABC feature of aconventional night vision device operating the conventional night visiondevice may drastically lose resolution so that the user of the device isno longer able to discern details of the viewed scene which would bediscernible were they viewed under darker field conditions in which ABCwere not applying.

BSP is provided in conventional night vision devices by decreasing thefield voltage provided to the photocathode. This voltage reductionhappens in the conventional power supply circuit because when a brightobject appears in the viewed scene a large number of photons will beincident on an area of the photocathode, and the high impedance of thephotocathode in combination with a high resistance value circuit elementcreates a greater voltage drop under these high current conditions. Thelarge number of photoelectrons provided by the photocathode under theseconditions represent a current flow increasing in magnitude withincreasing light levels in the viewed field, such that the combinedimpedance of the photocathode and circuit element causes a decrease inthe voltage level effective at the photocathode to move these electronsto the microchannel plate(s).

Recalling FIG. 9, it will be noted that the circuit architecture of theprior art uses two transformers, which are relatively large and heavycomponents of the circuit. Further, is seen that a typical conventionalcircuit architecture for a power supply of a night vision deviceprovides a high-value resistor (generally 1-18 G-ohm) to the output ofthe photocathode voltage multiplier, and a clamping circuit consistingof a voltage source and a low-leakage, high-voltage diode. Asphotocathode current flows through the high-value resistor, thephotocathode voltage will decrease linearly until it reaches a voltageequal to the voltage source (plus the high-voltage low-leakage diodevoltage drop). See FIG. 11 for a graphical illustration of this BSPvoltage relationship at the photocathode. This voltage is commonlyreferred to as a clamp voltage, and is typically between 30 and 40 voltsD.C.

This conventional method of BSP also has a disadvantage of decreasedresolution for the I² T. The reduced electrostatic field between thephotocathode and the microchannel plate(s) input causes a reducedresolution for the tube. That is, photoelectrons liberated from thephotocathode are not moved to the microchannel plate(s) as effectively,and may not be liberated to reach the microchannel plate(s) at all. Thisis because photoelectrons must overcome a surface potential barrier atthe photocathode in order to be liberated into free space (i.e., into avacuum within the image intensifier tube), and to be moved by theprevailing electrostatic field to the input of the microchannelplate(s). As the voltage applied to the photocathode decreases (againviewed statistically), some photoelectrons will not be able to overcomethis surface potential barrier and will not be liberated into freespace. The image information represented by these trapped photoelectronswill be lost from the image provided by the I² T to the user of thenight vision device.

SUMMARY OF THE INVENTION

In view of the deficiencies of the conventional related technology, itwould be desirable and is an object of this invention to provide animage intensifier tube which overcomes or reduces the severity of atleast one deficiency of the conventional technology.

Further, in view of the deficiencies of the conventional relatedtechnology, it would be desirable and is an object of this invention toprovide a night vision device which overcomes or reduces the severity ofat least one deficiency of the conventional technology.

An additional object for this invention is to provide an imageintensifier tube which does not employ an ion barrier film on the inputface of its microchannel plate(s), and which also does not suffer fromion damage to the photocathode of the image intensifier tube to theextent experienced in conventional image intensifiers in night visiondevices.

Yet another object for this invention is to provide a night visiondevice having such an image intensifier tube along with a power supplywhich provides both ABC and BSP functions substantially withoutdecreasing either the differential voltage applied to the microchannelplates(s) of the tube or of decreasing the voltage applied to thephotocathode of the tube.

Accordingly, it is an object for this invention to provide such a nightvision device which has improved resolution under ABC and BSP operatingconditions, and which also provides an improved gain in the imageintensifier tube.

Additionally, it would be desirable to provide such a power supply foran I² T which, for purposes of BSP under extremely bright-fieldconditions, does reduce microchannel plate differential voltage, but toa much lower extent of voltage reduction than was used in conventionalnight vision devices for purposes of ABC.

Still another desirable feature of an improved night vision device, andpower supply for an I² T of a night vision device, would be to provide aconstant voltage for connection to the photocathode of the I² T.

An advantage for such an improved power supply could be realized if theconstant-voltage to the photocathode of the image intensifier tube weregated on and off at a constant duty cycle over part of the current rangefor the phosphorescent screen of the tube, and were gated at a dutycycle inversely related to current level at this phosphorescent screenof the tube over a portion of the current level range indicative of aneed for an ABC function.

Accordingly, is a specific object for this invention to provide a nightvision device having an image intensifier tube with a microchannel platenot having an ion barrier film on its inlet face (and therefore, havingmicrochannels which are open to receive photoelectrons directly from aphotocathode of the tube) and with an improved power supply for the I² Toperating the tube in such a way as to decrease or eliminate ionbombardment damage to the photocathode of the tube.

To this end, the present invention according to one aspect provides anight vision device comprising: an objective lens receiving light from ascene being viewed and directing this light to an image intensifiertube, the image intensifier tube providing a visible image of the scenebeing viewed, and an eyepiece lens providing this visible image to auser of the night vision device; the image intensifier tube including aphotocathode receiving photons from the scene and releasingphotoelectrons in a pattern replicating the scene, a microchannel platehaving microchannels opening in the direction of the photocathode toreceive the photoelectrons and responsively providing a shower ofsecondary emission electrons in a pattern replicating the scene, and ascreen receiving the shower of secondary emission electrons andproducing a visible image replicating the scene; the night vision devicefurther including a source of electrical power at a selected voltagelevel, and a power supply circuit receiving the electrical power at theselected voltage level to responsively provide electrical power athigher voltage levels to the photocathode, to opposite faces of themicrochannel plate, and to the screen, the power supply circuitproviding a determined voltage level for connection to the photocathodeand continually connecting and disconnecting the determined voltagelevel to and from the photocathode.

Advantages which derive from this invention include the provision of amicrochannel plate which does not have an ion barrier film, and yetwhich is usable in an image intensifier tube with greatly reduced ionbombardment of the photocathode and reduced photocathode damage levelscompared to conventional image intensifier tubes. Another advantage isthat the improved power supply for an I² T results in a night visiondevice which does not experience the loss of resolution in bright fieldconditions which is common with conventional night vision devices. Infact, resolution, signal-to-noise ratio, and fixed pattern noise of theimage intensifier are all preserved at desirably high levels throughoutthe ABC and BSP operations of the tube and power supply--which is notthe case with conventional I² T power supplies.

Additionally, mean time between failures for the power supply may beimproved in comparison to conventional power supplies because partscounts may be reduced. The usable service life of an image intensifiertube operated by a power supply as set out in this disclosure isextended greatly. This improvement service life is obtained also withimage tubes having filmed microchannel plates. Moreover, the inventionprovides an image intensifier tube which does not have a filmedmicrochannel plate, which provides improved gain and improved signal tonoise ratio as a result, and which still has an improved service lifeover conventional image intensifier tubes, which conventional tubesprovide a lower gain.

A further advantage of the present inventive power supply is that thephotocathode and microchannel plate at all times enjoy operation at fulldesign voltage level (except for extreme cases of BSP in which voltageto the MCP is reduced slightly). Effectively, gating of the photocathodevoltage "simulates darkness" during that portion of the gating cycle inwhich voltage is off, and keeps the components of the I² T operatingunder the ideal conditions of low average current densities that theywere designed for.

Still another advantage results from the reduced electron energynecessary to introduce electrons into the microchannels of the MCP incomparison to conventional image intensifier tubes. Because themicrochannels of an image intensifier tube embodying the presentinvention are open in the direction facing the photocathode, no barrieris present to restrict their entry, and the photoelectrons haveessentially no barrier to overcome. This is in contrast to conventionalproximity focused image intensifier tubes, which have an ion barrier onthe input side of the MCP. Electrons must effectively penetrate the ionbarrier to get into the microchannels of the conventional imageintensifier tube. Thus, the voltage applied to the photocathode of animage tube operated according to the invention can be lowered, and stillprovide an adequate flow of photoelectrons to the microchannel plate.This advantage allows use of a smaller power supply.

Other objects, features, and advantages of the present invention will beapparent to those skilled in the art from a consideration of thefollowing detailed description of a preferred exemplary embodimentthereof taken in conjunction with the associated figures which willfirst be described briefly.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a schematic representation of a night vision device embodyingthe present invention;

FIG. 2 shows an I² T in longitudinal cross section, with an associatedpower supply embodying the present invention;

FIG. 2a is a greatly enlarged fragmentary cross sectional view of aportion of the microchannel plate of the I² T seen in FIG. 2;

FIG. 3 is a schematic representation of an improved power supply for anI² T embodying the present invention;

FIGS. 4-8 respectively provide graphical representations of photocathodepeak voltage, duty cycle, voltage wave form, microchannel plate voltage,and I² T output brightness; and

FIGS. 9-11 respectively provide a schematic circuit illustration, andgraphical representations of microchannel plate voltage and ofphotocathode voltage for a conventional I² T power supply.

DETAILED DESCRIPTION OF AN EXEMPLARY PREFERRED EMBODIMENT OF THEINVENTION

While the present invention may be embodied in many different forms,disclosed herein is a specific exemplary embodiment that illustrates andexplains the principles of the invention. It should be emphasized thatthe present invention is not limited to the specific embodimentillustrated.

Referring first to FIG. 1, there is shown schematically the basicelements of one version of a night vision device 10 of the lightamplification type. Night vision device 10 generally comprises a forwardobjective optical lens assembly 12 (illustrated schematically as afunctional block element having a lens depicted in dashed lines--andwhich may include one or more lens elements). This objective lens 12focuses incoming light from a distant scene [which may be a day-timescene illuminated with full day light (as will be explained) or may be anight-time scene illuminated with only star light or with infrared lightfrom another source] through the front light-receiving end surface 14aof an I² T 14 (as will be seen, this surface 14a is defined by atransparent window portion of the tube--to be further described below).As was generally explained above, the I² T provides an image at lightoutput end 14b in phosphorescent yellow-green visible light, which imagereplicates the viewed or night-time scene.

Hereinafter, no distinction is maintained between the cases in which thescene is visible with natural vision to the user of the device, and thecases in which the scene is totally invisible because it is illuminatedonly with star light or other infrared light. The device 10 can providea visible image replicating the scene for the user at both of theseextremes, and at all illumination levels between these extremes. Again,a night time scene would generally be not visible (or would be onlypoorly visible) to a human's natural vision. This visible image ispresented by the device 10 via an eye piece lens illustratedschematically as a single lens 16 producing a virtual image of the rearlight-output end of the tube 14 at the user's eye 18.

More particularly now viewing FIG. 2, it is seen that I² T 14 includes aphotocathode (PC) 20 which is responsive to photons of infrared light toliberate photoelectrons, a microchannel plate (MCP) 22 which receivesthe photoelectrons in a pattern replicating the night-time scene, andwhich provides an amplified pattern of electrons also replicating thisscene, and a display electrode assembly 24. In the present embodimentthe display electrode assembly 24 may be considered as having analuminized phosphor coating or phosphor screen 26. When this phosphorcoating is impacted by the electron shower from microchannel plate 22,it produces a visible image replicating the pattern of the electronshower. Because the electron shower in pattern intensity stillreplicates the scene viewed via lens 12, a user of the device caneffectively see in the dark, viewing a scene illuminated by only starlight or other low-level or invisible infrared light. A transparentwindow portion of the assembly 24, to be further described below,conveys the image from screen 26 outwardly of the tube 14 so that it canbe presented to the user 18. The window portion 24a may be plain glass,or may be fiber optic, as depicted in FIG. 2. Those ordinarily skilledwill understand that a fiber optic output window 24a inverts the imageprovided by the screen 26.

Still more particularly considering FIGS. 2 and 2a, the microchannelplate 22 is located just behind photocathode 20, with the microchannelplate 22 having an electron-receiving face 28 and an oppositeelectron-discharge face 30. This microchannel plate 22 further containsa plurality of angulated microchannels 32 which open on theelectron-receiving face 28 and on the opposite electron-discharge face30. Microchannels 32 are separated by passage walls 34. At least aportion of the surface of the passage walls 34 bounding the channels 32is defined by a material 34a, which is an emitter of secondaryelectrons. In this case, and in contrast to conventional imageintensifier tubes of the illustrated proximity focused type, themicrochannel plate does not carry an ion barrier on input face 28.Accordingly, the microchannels 32 open on both faces of the plate 22,and are not obstructed at their inlet ends (i.e., at the left endviewing FIGS. 2 and 2a, which is disposed toward photocathode 20) by anion barrier film. Still viewing FIGS. 2 and 2a, it is seen that eachface 28 and 30 carries a conductive electrode layer 28a and 30a,respectively. These conductive electrode layers may be metallic, or maybe formed of other conductive material so as to distribute anelectrostatic charge over the respective faces of the microchannel plate22. These electrode coatings do not span across the openings of themicrochannels 32, and do not close the openings of these microchannels,so that these microchannels are open in a direction disposed toward thephotocathode (again, leftwardly, viewing FIGS. 2 and 2a).

The display electrode assembly 24, generally has a conductive coatedphosphor screen 26, is located behind microchannel plate 22 withphosphor screen 26 in electron line-of-sight communication with theelectron-discharge face 30 of the MCP 22. The display electrode assembly24 is typically formed of an aluminized phosphor screen 26 deposited onthe vacuum-exposed surface of the optically transparent material ofwindow portion 24a. The focusing eye piece lens 16 is located behind thedisplay electrode assembly 24 and allows an observer 18 to view acorrectly oriented image corresponding to the initially receivedlow-level image.

As will be appreciated by those skilled in the art and also viewing nowFIG. 2, the individual components of I² T 14 are all mounted andsupported in a tube or chamber (to be further explained below) havingforward and rear transparent plates cooperating to define a chamberwhich has been evacuated to a low pressure. This evacuation allowselectrons liberated into the vacuum free-space within the tube to betransferred between the various components by prevailing electrostaticfields without atmospheric interference that could possibly decrease thesignal-to-noise ratio. Because of the close proximity of the componentsof this type of image intensifier tube, it is referred to as a"proximity focused" type of tube.

As indicated above, photocathode 20 is mounted immediately behindobjective lens 12 on the inner vacuum-exposed surface of the windowportion of the tube and before microchannel plate 22. Typically, thisphotocathode 20 is a circular disk-like structure having a predeterminedconstruction of semiconductor materials, and is mounted on a substratein a well known manner. Suitable photocathode materials are generallysemiconductors such as gallium arsenide; or alkali metals, such ascompounds of sodium, potassium, cesium, and antimony (commerciallyavailable as S-20), carried on a readily available transparentsubstrate. A variety of glass and fiber optic substrate materials arecommercially available.

Considering in somewhat greater detail the operation of the I² T 14, itis seen that in response to photons 36 entering the forward end of nightvision device 10 and passing through objective lens 12, photocathode 20has an active surface 20a from which are emitted photoelectrons innumbers proportionate to and at locations replicative of the receivedoptical energy of the night-time scene being viewed. Again, and ingeneral, the image received will be too dim to be viewed with humannatural vision, and may be entirely or partially of infrared radiationwhich is invisible to the human eye. It is thus understood that theshower of photoelectrons emitted from the photocathode arerepresentative of the image entering the forward end of I² T 14. Thepath of a typical photoelectron emitted from the photon input point onthe photocathode 20 is represented in FIG. 1 by dashed line 40.

Photoelectrons 40 emitted from photocathode 20 gain energy through anelectric field of predetermined intensity gradient established betweenphotocathode 20 and electron-receiving face 28, which field gradient isprovided by power source 42. Typically, power source 42 will apply anelectrostatic field voltage on the order of 200 to 800 volts to create afield of the desired intensity. After accelerating over a distancebetween the photocathode 20 and the input surface 28 of the microchannelplate 22, these photoelectrons 40 enter microchannels 32 of microchannelplate 22. As will be discussed in greater detail below, thephotoelectrons 40 are amplified by emission of secondary electrons toproduce a proportionately larger number of electrons upon passagethrough microchannel plate 22. This amplified shower ofsecondary-emission electrons 44, also accelerated by a respectiveelectrostatic field generated by power source 46, then exitsmicrochannels 32 of microchannel plate 22 at electron-discharge face 30.

Once in free space again, the amplified shower of photoelectrons andsecondary emission electrons is again accelerated in an establishedelectrostatic field provided by power source 48. This field isestablished between the electron-discharge face 30 and display electrodeassembly 24. Typically, the power source 48 produces a field on theorder of 3,000 to 7,000 volts, and more preferably on the order of 6,000volts in order to impart the desired energy to the multiplied electrons44.

The shower of photoelectrons and secondary-emission electrons 44 (thoseordinarily skilled in the art will know that considered statistically,the shower 44 is almost or entirely devoid of photoelectrons and is madeup entirely or almost entirely of secondary emission electrons.Statistically, the probability of a photoelectron avoiding absorption inthe microchannels 32 is low). However, the shower 44 is several ordersof magnitude more intense than the initial shower of photoelectrons 40,but is still in a pattern replicating the image focused on photocathode20. This amplified shower of electrons falls on the phosphor screen 26of display electrode assembly 24 to produce an image in visible light.

Viewing FIG. 2 in greater detail, the I² T 14 is seen to include atubular body 50, which is closed at opposite ends by a frontlight-receiving window 52, and by a rear fiber-optic image output window54. The window 54 defines the light output surface 14b for the tube 14,and carries the coating 26, as will be further described. As isillustrated in FIG. 2, the rear window 54 may be an image-inverting type(i.e., with optical fibers bonded together and rotated 180° between theopposite faces of this window 54 in order to provide an erect image tothe user 18. The window member 54 is not necessarily of such invertingtype. Both of the windows 52 and 54 are sealingly engaged with the body50, so that an interior chamber 56 of the body 50 can be maintained at avacuum relative to ambient. The tubular body 50 is made up of pluralmetal rings, each indicated with the general numeral 58 with analphabetical suffix added thereto (i.e., 58a, 58b, 58c, and 58d) as isnecessary to distinguish the individual rings from one another.

The tubular body sections 58 are spaced apart and are electricallyinsulated from one another by interposed insulator rings, each of whichis indicated with the general numeral 60, again with an alphabeticalsuffix added thereto (i.e., 60a, 60b, and 60c). The sections 58 andinsulators 60 are sealingly attached to one another. End sections 58aand 58d are likewise sealingly attached to the respective windows 52 and54. Those ordinarily skilled in the pertinent arts will know that thebody sections 58 are individually connected electrically to a powersupply 62 (which provides sources 42, 46, and 48, as described above),and which is effective during operation of the I² T 14 to maintain anelectrostatic field most negative at the section 58a and most positiveat the section 58d.

Further viewing FIG. 2, it is seen that the front window 52 carries onits rear surface within the chamber 56 the photocathode 20. The section58a is electrically continuous with the photocathode by use of a thinmetallization (indicated with reference numeral 58a') extending betweenthe section 58a and the photocathode 20. Thus, the photocathode by thiselectrical connection and because of its semi-conductive nature, has anelectrostatic charge distributed across the areas of this disk-likephotocathode structure. Also, a conductive coating or layer 28a, 30a isprovided at each of the opposite faces 28 and 30 of the microchannelplate 22 (as is indicated by arrowed numerals 28a and 30a). Power supply46 is conductive with these coatings by connection to housing sections58b and 58c. Finally, the power supply 48 is conductive with aconductive layer or coating (possibly an aluminum metallization, asmentioned above) at the display electrode assembly 24 by use of ametallization also extending across the vacuum-exposed surfaces of thewindow member 54, as is indicated by arrowed numeral 54a.

It should be noted in considering the description below of the structureand operation of the power supply 62, that the term "image intensifiertube" is used in a generic sense. Those ordinarily skilled in thepertinent arts will appreciate that the tube being powered may beconfigured as an electron multiplier tube in which the output is anelectrical signal rather than a visible image. Also, the tube beingpowered may be of the photodetector, phosphorescence detector, orscintillation detector type, in which the output is also an electricalsignal rather than a visible image. Such tubes are generally used, forexample, to detect a phosphorescent response in a chemical reagentexposed to exciting light of another color or wavelength, or in adetector for high-energy events having as a result of their occurrencethe production of a small number of photons (i.e., as few as one photonper event).

Such application of tubes having a photocathode and a dynode (either ofmicrochannel plate configuration with many dynodes, or of anotherconfiguration with one or more dynodes) may experience some or all ofthe difficulties in operation which are described above in the contextof night vision devices. Accordingly, it will be appreciated that apower supply embodying principles of this invention may be used in suchapplications.

Considering now FIG. 3, it is seen that the power supply 62 includes apower source, which in this case is illustrated as a battery 64. It willbe appreciated that a battery 64 is generally used as the power sourcefor portable apparatus, such as night vision devices. However, theinvention is not limited to any particular power source. For example, aregulated line-power source could be used to provide input power to apower supply implementing and embodying the principles of the presentinvention. Considered generally, the power supply 62 includes threevoltage multipliers or voltage converters, respectively indicated withthe numerals 66, 68, and 70. The voltage converter 66 for thephotocathode 20 includes two converters of differing voltage level, andindicated with the numerals 66a and 66b (note that the converter 66bprovides a voltage level which is positive with respect to the face 28of MCP 22, while converter 66a provides a voltage level which isnegative relative to the face 28 of the MCP 22.

A tri-stable switching network 72 switches controllably betweenalternative positions either conducting the photocathode 20 to voltageconverter 66a, to an open circuit position, or to voltage converter 66b,all via the conductive connection 72a. A duty cycle control 74 controlsthe switching position of the switching network 72, and receives asinputs a square wave gating trigger signal from an oscillator 76, and acontrol signal via a conductor 78 from an ABC/BSP control circuit 80. Itwill be appreciated that the switching network 72 may be configured toswitch (i.e., to toggle) between voltage sources 66a and 66b withouthaving an open-circuit condition. This alternative would yieldessentially a square-wave voltage on the graph of FIG. 6.

Power supply to the microchannel plate 22 (that is, to the conductivelayers or metallizations 28a and 30a) is effected from the voltageconverter 68 via connections 68a and 68b. Interposed in connection 68ais a series element 82, which in effect is a controllably variableresistor. A high-voltage MOSFET, for example, may be used for element82, and the resistance of this element is controlled over a connection82a by a regulator circuit 84. Regulator circuit 84 receives a feed backcontrol signal from a summing junction 86, which receives an input fromconductor 88 via a level-adjusting resistor 90, and also receives aninput via conductor 92 from the ABC/BSP control circuit 80. Conductor 88also provides a feed back signal of the voltage level applied to theinput face 28 (i.e., at metallization 28a) of the microchannel plate 22into the voltage converter circuit 66. Note that this conductor 88provides a reference voltage level of microchannel plate voltage on face28 (electrode 28a), not a signal of photocathode voltage level.

The voltage converter 70 has connection to the screen 26 via aconnection 70a, and provides a feed back of screen current level intoABC/BSP control circuit via conductor 94. Energy flow in the circuit 62is provided by an oscillator 96 and coupled transformer 98, with pluraloutput windings 98a providing energy input to voltage converters 66 and70, and a conductor 100 providing energy to voltage converter 68. It isnoted that the circuit 62 requires only the single transformer 98, whichadvantageously reduces cost, size, weight, and parts count for the powersupply; and also improves reliability for the power supply and nightvision device 10. The oscillator 96 receives a control feed back via aregulator 102 and a feed back circuit 104, having an input from afeedback winding 98b of transformer 98.

Having considered the structure of the circuit 66, attention may now begiven to its operation, and the cooperation of this circuit operationwith the operation of the I² T 14. Attention now to FIGS. 4-8, withattention first to FIG. 4, shows that the most negative voltage levelproduced by voltage converter 66a for application by power supplycircuit 66 to the photocathode 20 of the tube 14 is always constant at aselected voltage level. Comparing this FIG. 4 to the voltage curve ofFIG. 11 reveals that the prior art teaches to vary the voltage appliedto the photocathode in order to provide a BSP function. However, FIG. 5shows that the power supply circuit 66 provides a BSP function bykeeping the voltage applied to the photocathode 20 constant (recallingFIG. 4) while gating connection of the photocathode between connectionto this constant voltage source (i.e., about -800V), to an open circuit(i.e., voltage off), and to a lower voltage (i.e., relatively morepositive relative to the face 28 of MCP 22--about +30V) provided byvoltage converter 66b (simulating darkness for the photocathode 20).When the photocathode 20 is connected to voltage source 66b (i.e., to asource of about +30 volts relative to the face 28 of MCP 22), thiscondition might be considered a "hard turn off" for the photocathode.Under this condition, the photocathode is not responsive to photonsreceived from the scene being viewed. This gating function is carried onat a constant cyclic rate and cycle interval, while varying the dutycycle of the applied constant voltage preferably as a function ofcurrent level sensed at screen 26 (i.e., by feed back over conductor94).

It should be noted, however, that this voltage gating function forphotocathode 20 can be carried out with respect to other parameters ofoperation of the image intensifier tube 14. For example, an alternativeway of controlling the gating function would be to use the current levelat face 30 i.e., at electrode 30a) as a controlling parameter.

FIG. 5 shows that over a range of screen current indicated with thenumeral 106, the duty cycle of the applied constant voltage to thephotocathode 20 is fixed at 100%. However, at screen current levelsabove a selected level, the duty cycle progressively ramps downsubstantially linearly to a low level of essentially 10⁻⁴ % as afunction of increasing screen current, as is indicated by numeral 108.It will be noted that FIGS. 5 and 7 are drawn to the same scale ofscreen current along the abscissa of the of the graph, and that thesegraphs are arranged one vertically above the other for the reader'sconvenience in understanding the relationship of photocathode gatingduty cycle to voltage applied to the microchannel plate 22.

Returning attention to FIG. 5 and with attention now to FIG. 6 it isseen that for screen current levels above that at which the duty cyclefor gating of the constant voltage to the photocathode 20 reaches itslowest value, an additional function of BSP is provided by decreasingthe voltage applied to the microchannel plate 22 (indicated by theencircled portion 110 of FIG. 7). It will be noted that for all screencurrent levels lower than those necessary to initiate this BSPprotection function, the voltage applied across the microchannel plate20 is a constant. Thus, the microchannel plate 22 operates at itsdesigned voltage differential across electrodes 28a and 30a. Thereduction of voltage level applied across the microchannel plate 20 iseffected by action of the series element 82 increasing its resistanceunder control of MCP regulator 84. As noted this regulator 84 receives asummed input from the conductor 88 via the level adjusting resistor 90,and from the ABC/BSP control circuit 80, which itself is responsive tothe level of current sensed at screen 26 by conductor 94.

Comparing this operation of power supply circuit 62 to the operation ofthe conventional power supply discussed above with reference to FIGS.9-11, and viewing particularly FIG. 10, it is seen that the power supply62 avoids the problem of loss of resolution for an I² T, which is causedin the conventional power supplies by operation with too low adifferential voltage across a microchannel plate.

The voltage wave form of FIG. 6 might be produced by a rapid increase oflight input such that the ABC function, and then the BSP functionoperate in succession. For purposes of illustrating this action, FIG. 6is also annotated with a time arrow, indicating that in this instancetime proceeds from left to right on the graph. It will be noted that theconstant voltage level gated to the photocathode 20 (i.e., from voltageconverter 66a) is substantially -800V, while the positive voltage levelfrom voltage converter 66b is about +30 volts relative to the face 28(electrode 28a) of the microchannel plate 22.

The reader should not be confused by the similarity in appearancebetween the graph of FIG. 10 and that of FIG. 5, they are illustratingdiffering values. FIG. 10 relates to conventional microchannel platevoltage, while FIG. 5 is voltage gating duty cycle to the photocathode20 as provided by the power supply 62.

In view of the above, attention now to FIG. 6 provides an understandingof the microchannel plate voltage level as the duty cycle for theapplication of the constant peak voltage seen in FIG. 4 is varied inresponse to changing light levels in the viewed scene, and in responseto the changes in screen current level for the I² T. FIG. 6 shows thatportion of the duty cycle operation corresponding to portions 108 and110 of FIGS. 5 and 7. Increasing light levels and increasing screencurrent levels go from left to right on the graph of FIG. 6. It will benoted that a portion of the graph of FIG. 6 is not shown (i.e., to theleft of that part shown). This portion which is not shown wouldcorrespond to section 106 of FIG. 5, and in this realm of operation theduty cycle is always 100%.

At the left side of the graph of FIG. 6, it is seen that the duty cycleis here slightly less than 100%, and that within the interval for eachduty cycle the voltage applied to photocathode 20 is initially the highconstant peak voltage indicated in FIG. 4 (i.e., indicated at numerals112), and then decays over a very short time interval at a naturalopen-circuit, capacitor-discharge rate (indicates at segments 114 of thevoltage curve). This voltage decay is actually a very small voltagebecause of the short time interval, and occurs because the virtualcapacitor existing between the photocathode 20 and the conductivemetallization on the front light-receiving face of the microchannelplate 22 (i.e., conductive coating 28a) is open-circuit when theswitching network 72 (recalling FIG. 3) is not conducting thephotocathode to neither voltage converter 66a or to voltage converter66b. This virtual capacitor is diagrammatically indicated on FIG. 3, andindicated with the character "C".

Next in each duty cycle, the network 72 conducts the photocathode tovoltage converter 66b (i.e., about positive 30 volts) which effectivelyreplicates darkness for the photocathode 20 by dropping the voltage asis indicated at voltage cutoff's 116 of FIG. 6. Effectively, thisdropping (i.e., more positive) voltage level for the photocathode 20 isa hard turn off. That is, when the applied voltage at the photocathode20 is about +30 volts relative to the face 28 of microchannel plate 22,then electrons will not flow from this photocathode to the microchannelplate in response to photon of light hitting the photocathode. Thisvoltage cutoff 116 is provided by having voltage converter 66b provide avoltage which is about 30 volts positive with respect to the voltageprovided at coating 28a on the front face of the microchannel plate 22by voltage converter 68.

Restated, it is seen that in essence when the photocathode 20 operates,it always operates substantially at the high constant peak voltage seenin FIG. 4. When the photocathode 20 is not operating, it is switched toa voltage which replicates a dark field for the photocathode (i.e., the+30 volts from voltage converter 66b). The photocathode 20 operated bythe power supply 62 of the present invention is switched betweenoperation at its designed voltage level and dark-field condition at aduty cycle which varies dependent upon the light intensity of the scenebeing viewed, as indicated by current flow at the screen 26. Thisfunction is carried out in accord with the duty cycle function indicatedin FIG. 5 in order to provide ABC. The result of this ABC operation isillustrated in FIG. 8, which indicates that over a broad range of inputlight levels, a substantially constant brightness for the imagepresented to a user of the night vision device 10 is achieved. At theleft-hand side of FIG. 8 is seen a linearly decreasing section of thebrightness curve from the image intensifier tube 14. This occurs withvery dim lighting levels, but the image intensifier tube 14 will stillprovide a usable image in at least a portion of this regime of itsoperation.

Returning to consideration of FIGS. 5 and 7, within section 108, theduty cycle is progressively decreased until it reaches it low level of10⁻⁴ % as a function of increasing screen current. If light level of theviewed scene continues to increase (indicative of a bright source in thescene), then the duty cycle maintains its low 10⁻⁴ % level, while thebright source protection function explained above is effected in section110, recalling FIG. 7.

Having explained the operation of power supply circuit 62 and itscooperation with the image intensifier tube 14, attention can now begiven to the effect this power supply operation is believed to have onthe migration of positive ions within an image intensifier tube. Thesepositive ions (which are generally atoms of hydrogen and other gasseswhich can never be entirely removed from the vacuum space 56) travel inthe direction opposite to the electron flow because of the electrostaticfields maintained within the tube 14. Thus, positive ions travel towardand bombard photocathode 20. However, in the present device, the voltageapplied to the photocathode is not always negative, is intermittent, andis positive part of the time. Thus, positive ions are believed to bemoved toward the photocathode less constantly than is conventionally thecase. Further, the positive ion bombardment of the photocathode 20appears to be much less damaging in the present inventive combination.For this reason, the MCP 22 of the present invention need not carry anion barrier film on the surface 28, and the number of photoelectronsreceived into the MCP 22 is improved by about 50% by this reason alone.Thus, a NVD embodying the present invention can provide a much betteramplification of light, and an improved operating life span for the tube14.

Those skilled in the art will appreciate that the embodiment of thepresent invention depicted and described herein and above is notexhaustive of the invention. For example, the ABC and BSP aspects of theinvention may be implemented separately of one another if desired. Thoseskilled in the art will further appreciate that the present inventionmay be embodied in other specific forms without departing from thespirit or central attributes thereof. Because the foregoing descriptionof the present invention discloses only an exemplary embodiment thereof,it is to be understood that other variations are recognized as beingwithin the scope of the present invention. Accordingly, the presentinvention is not limited to the particular embodiment which has beendescribed in detail herein. Rather, reference should be made to theappended claims to define the scope and content of the presentinvention.

I claim:
 1. A night vision device comprising:an objective lens receivinglight from a scene being viewed and directing this light to an imageintensifier tube, said image intensifier tube providing a visible imageof the scene being viewed, and an eyepiece lens providing this visibleimage to a user of the night vision device; said image intensifier tubeincluding a photocathode receiving photons from the scene and releasingphotoelectrons in a pattern replicating the scene, a microchannel platehaving microchannels opening in the direction of said photocathode toreceive the photoelectrons and responsively providing a shower ofsecondary emission electrons in a pattern replicating the scene, and ascreen receiving the shower of secondary emission electrons andproducing a visible image replicating the scene; said night visiondevice further including a source of electrical power at a selectedvoltage level, and a power supply circuit receiving said electricalpower at said selected voltage level to responsively provide electricalpower at plural higher voltage levels to each of: said photocathode, toopposite faces of said microchannel plate, and to said screen; saidpower supply circuit providing as one of said plural higher voltagelevels a determined voltage level which is available for connection tosaid photocathode; and said power supply circuit including a switchingmeans continually connecting and disconnecting said determined voltagelevel to and from said photocathode.
 2. The night vision device of claim1 wherein said power supply circuit also provides as one of said pluralhigher voltage levels another voltage level which is also available forconnection to said photocathode, and said switching means of said powersupply circuit continually switching said photocathode betweenconnection with said determined voltage level and alternative connectionwith said another voltage level.
 3. The night vision device of claim 1wherein said another voltage level is a positive voltage.
 4. The nightvision device of claim 3 wherein said switching means of said powersupply circuit sequentially connects said photocathode first to saiddetermined voltage level, and then to open circuit having no connectionto either said determined voltage level or to said another voltagelevel.
 5. A night vision device having an objective lens receiving lightfrom a scene being viewed and directing this light to an imageintensifier tube, said image intensifier tube providing a visible imageof the scene being viewed, and an eyepiece lens providing this visibleimage to a user of the night vision device;said image intensifier tubeincluding a photocathode receiving photons from the scene and releasingphotoelectrons in a pattern replicating the scene, a microchannel platehaving a multitude of microchannels which open without obstructiontoward said photocathode to receive the photoelectrons, saidmicrochannel plate responsively providing a shower of secondary emissionelectrons in a pattern replicating the scene, and a screen receiving theshower of secondary emission electrons and producing a visible imagereplicating the scene; said night vision device including a source ofelectrical power at a selected voltage level; and a power supply circuitreceiving said electrical power at said selected voltage level toresponsively provide plural higher voltage levels to each of: saidphotocathode, to opposite faces of said microchannel plate, and to saidscreen; wherein said power supply circuit further includes a voltageconverter circuit providing as one of said plural higher voltage levelsa determined voltage level which is available for connection to saidphotocathode, and said power supply circuit also providing anothervoltage converter circuit providing another voltage level which is alsoavailable to be connected to said photocathode, said another voltagelevel being relatively positive; and a switching device continuallyopening and closing connection of said determined voltage level to saidphotocathode and alternatingly connecting said another voltage level tosaid photocathode.
 6. The night vision device of claim 5 wherein saidswitching device in a sequential gating cycle and after connection ofsaid photocathode to said voltage converter circuit which is providing arelative negative voltage level to said photocathode, then connects saidphotocathode to open circuit in which said photocathode has connectionto neither one of said voltage converter circuit or to said anothervoltage converter circuit.
 7. A method of operating a night visiondevice, said night vision device having an objective lens receivinglight from a scene being viewed and directing this light to an imageintensifier tube, said image intensifier tube providing a visible imageof the scene being viewed, and an eyepiece lens providing this visibleimage to a user of the night vision device; said image intensifier tubeincluding a photocathode receiving photons from the scene and releasingphotoelectrons in a pattern replicating the scene, a microchannel platereceiving the photoelectrons and providing a shower of secondaryemission electrons in a pattern replicating the scene, and a screenreceiving the shower of secondary emission electrons and producing thevisible image replicating the scene; said method including stepsof:providing both a constant negative voltage level available to beswitched to said photocathode, and also providing a voltage level whichpositive relative to a first face of the microchannel plate and which isalso available to be switched to said photocathode; continuouslyalternating connection of said photocathode between said constantnegative voltage level and said relative positive voltage level; andproviding said microchannel plate with a multitude of microchannelswhich are open and unobstructed in a direction toward said photocathodeto receive photoelectrons liberated by said photocathode.
 8. The methodof claim 7 further including the steps of providing for said connectionof said photocathode between said negative and said relatively positivevoltage levels to be varied in a variable duty cycle, and maintainingsaid variable duty cycle at substantially 100% over a first range ofscreen current, and progressively decreasing said duty cycle from 100%to a lower level over a second range of screen current.
 9. The method ofclaim 8 including the selection of said lower level for said duty cycleto be substantially 10⁻⁴ %.
 10. The method of claim 9 further includingthe steps of providing a voltage differential across said microchannelplate, and decreasing said voltage differential across said microchannelplate after said variable duty cycle has decreased to a selected level.11. A method of operating an image intensifier tube, said imageintensifier tube including a photocathode receiving photons andreleasing photoelectrons, a microchannel plate receiving thephotoelectrons and providing a shower of secondary emission electrons,and a screen receiving the shower of secondary emission electrons andproducing a visible image; said method including steps of:providing saidmicrochannel plate with a multitude of microchannels open andunobstructed in a direction toward said photocathode; providing aconstant negative voltage level available to be switched to saidphotocathode; providing a voltage level which is positive with respectto a first face of the microchannel plate; and continuously switchingsaid photocathode alternatingly from said constant voltage level to saidrelative positive voltage.
 12. The method of claim 11 further includingthe steps of: providing for said switching to be performed in a variableduty cycle, maintaining said variable duty cycle at substantially 100%over a first range of screen current, and progressively decreasing saidduty cycle from 100% to a lower level over a second range of screencurrent.
 13. The method of claim 12 further including the step ofselecting said lower level to be substantially 10⁻⁴ %.
 14. The method ofclaim 13 further including the steps of providing a voltage differentialacross said microchannel plate, and decreasing said voltage differentialacross said microchannel plate to a lower level after said variable dutycycle has reached a selected low level.
 15. The method of claim 14wherein said voltage differential across said microchannel plate isreduced after said variable duty cycle is decreased to substantially10⁻⁴ %.
 16. A method of operating an image intensifier tube, said imageintensifier tube including a photocathode receiving photons andreleasing photoelectrons, a microchannel plate receiving thephotoelectrons and providing a shower of secondary emission electrons,and a screen receiving the shower of secondary emission electrons andproducing a visible image; a power supply providing selected voltagelevels to each of said photocathode, to opposite faces of saidmicrochannel plate, and to said screen; said method including stepsof:providing said microchannel plate with a multitude of microchannelsopening and unobstructed in a direction toward said photocathode;utilizing said power supply to provide a respective voltage level foreach of opposite faces of said microchannel plate and for said screen;utilizing said power supply to provide a negative voltage levelavailable to be switched to said photocathode, and another relativepositive voltage level which is also available to be switched to saidphotocathode and which is positive relative to a voltage level providedto a first of said opposite faces of said microchannel plate;continuously switching connection of said photocathode alternatinglyfrom said negative voltage level to said relative positive voltagelevel.
 17. The method of claim 16 further including the steps ofswitching said voltage levels to said photocathode in a variable dutycycle, and maintaining said variable duty cycle at substantially 100%over a first range of screen current, and progressively decreasing saidduty cycle from 100% to a lower level over a second range of screencurrent.
 18. The method of claim 17 wherein said lower level is selectedto be substantially 10⁻⁴ %.
 19. A dynode-array light responsive device,said device comprising:a tube having a photocathode, a microchannelplate defining plural dynodes, and an output electrode; saidphotocathode responding to photons of light to liberate photoelectronswithin said tube, said microchannel plate having plural microchannelswhich are open and unobstructed in a direction toward the photocathodeto received photoelectrons liberated by said photocathode; said pluralmicrochannels each responsively providing secondary emission electronsto define respective dynodes of an array, and thereby providing a showerof secondary emission electrons to said output electrode; a source ofelectrical power at a selected voltage level, and a power supply circuitreceiving electrical power at said selected voltage level toresponsively provide plural higher voltage levels respectively to saidphotocathode, to said pair of opposite faces of said microchannel plate,and to said output electrode; said power supply circuit furtherproviding a positive voltage level, and said power supply circuitincluding a switching device continuously switching connection of saidphotocathode to and from said higher voltage level alternatingly withconnection of said photocathode to and from connection with saidpositive voltage level.
 20. A method of operating an image intensifiertube, said image intensifier tube providing a shower of electronsreplicating a scene being viewed; said image intensifier tube includinga photocathode liberating photoelectrons in response to photos from ascene, a microchannel plate receiving the photoelectrons andresponsively providing a shower of secondary emission electrons in apattern replicating the scene, and an output electrode receiving theshower of secondary electrons and responsively providing imageinformation; said method including steps of:providing said microchannelplate with plural microchannels opening without obstruction toward thephotocathode; providing a negative voltage level available to beswitched to said photocathode; providing a relative positive voltagelevel; in a duty cycle continually toggling connection of saidphotocathode between said negative voltage level and said relativepositive voltage level.